A low-noise semiconductor light emitting diode having the energy band structure shown in FIG. 27 is disclosed in the Journal of Optical Society of America, B, Volume 4, Number 10, October 1987, pages 1663-1666 (hereinafter referred to as JOSA). Although in this publication an AlGaAs light emitting diode is described, it is suggested that a similar structure is applicable to a laser diode. Therefore, an InP laser diode having the energy band structure shown in FIG. 27 is described. In FIG. 27, reference numeral 1 designates the band gap energy of an n type InP cladding layer, 2 the band gap energy of an InGaAsP active layer, 3 the band gap energy of a p type InP cladding layer, and 4a the band gap energy of an intrinsic (hereinafter referred to as i type) InP layer. The thickness L of the i type InP layer 4a is larger than the diffusion length Ln of electrons in the i type InP layer 4a, for example, L=2Ln.
Since the thickness of the i type InP layer 4a is larger than the diffusion length of the electrons in the i type InP layer 4a, electrons accumulate in that layer 4a and there is Coulomb repulsion between the accumulated electrons. As a result, the electrons are injected into the active layer 2 regularly, not at random.
Therefore, laser light is generated regularly. The square of the fluctuation of the number of photons &lt;.DELTA.n.sup.2 &gt;=&lt;n.sup.2 &gt;-&lt;n&gt;.sup.2 is smaller than &lt;n&gt;, where n is the number of photons and &lt;&gt; means the average, when electrons are injected into the active layer at random and coherent light is generated. Since the fluctuation in the number of photons is reduced, a low-noise laser diode is realized.
According to JOSA, the relationship between a current IL flowing in the i type InP layer 4a and a voltage VL applied to that layer 4a is EQU IL=(9/8)(.epsilon..mu..sub.n /L.sup.3)AVL (1)
wherein .epsilon. is the dielectric constant, .mu..sub.n is electron mobility in the i type InP layer 4a, and A is the area in which the current flows. A is equal to w (stripe width).times.d (resonator length). When the laser light is connected to a photodetector through an optical fiber, a value F obtained by dividing the square of fluctuation of photon number by the average photocurrent is EQU F=1+.eta.L..eta.C..eta.D (8kT/VL-1) (2)
where .eta.C is the coupling efficiency of the laser light to the optical fiber, .eta.D is the quantum efficiency of the photodetector, k is Boltzmann's constant, T is the absolute temperature, and .eta.L is the quantum efficiency of the laser diode. The quantum efficiency .eta.L of the laser diode is ##EQU1## where 1n is the natural logarithm, .eta.i is the internal quantum efficiency, .alpha.in is the internal loss, Rf is the reflectivity at the front facet of the resonator, and Rr is the reflectivity at the rear facet of the resonator.
According to Applied Physics Letters, Volume 60, pages 3217-3219 (1992) (hereinafter referred to as APL), an InGaAsP series laser diode with a threshold current of 1 mA is realized by employing a strained quantum well structure as an active layer. In this laser diode, the internal loss .alpha.in is 10 cm.sup.-1. In InGaAsP, the internal quantum efficiency .eta.i is 0.8 as described in IEEE Journal of Quantum Electronics, Volume QE-24, pages 29-35 (1988). Assuming that Rf=1%, Rr=90%, .eta.C=0.65, and .eta.D=0.90, .eta.L..eta.C..eta.D becomes 0.32. In this case, the operating current at a light output of 5 mW is 11 mA.
In order to obtain an i type semiconductor in which the Fermi energy in the thermal equilibrium state is in the center of the band gap, InP for example, having a band gap energy of 1.35 eV is doped with Fe having an impurity level at a position of 0.65 eV from the conduction band edge. According to "Properties of Indium Phosphide" INSPEC (1990) (hereinafter referred to as INSPEC), the resistivity .rho. of InP doped with Fe is 2.1-4.6.times.10.sup.7 .OMEGA.cm. Fe produces acceptor levels. According to INSPEC, doping to 5.times.10.sup.16 /cm.sup.3 gives an electron diffusion length (Ln) of 0.75 .mu.m. Assuming that L=2Ln=1.5 .mu.m, w=1.5 .mu.m, and d=300 .mu.m, the resistivity RL of the InP layer doped with Fe is 7.times.10.sup.8 .OMEGA.cm when .rho. is 2.1.times.10.sup.7 .OMEGA.cm. In this state, when a current of 11 mA flows, 7.7.times.10.sup.6 V (VL) is applied to the InP layer, so that the value F of the equation (2) is 0.68, that is, smaller than the 0.899, of the light emitting diode described in JOSA in which an i type AlGaAs layer is interposed between an n type AlGaAs cladding layer and a GaAs active layer, a further reduction in noise is achieved.
However, an intrinsic semiconductor layer has a high resistivity. For example, in the prior art InP series laser diode described above, the power consumption of the Fe doped InP layer, i.e., the intrinsic semiconductor layer, is 84.7.times.10.sup.3 W. Assuming that the thermal resistivity is 30.degree. C./W, the temperature rise is as high as 2,540,000.degree. C. Such a high temperature rise melts the crystal.